Plugging switch mechanism



p 20, 1955 w. H. ELLIOT ET AL PLUGGING SWITCH MECHANISM Filed Jan. 9, 1952 2 Sheets-Sheet l Ilu INVENTORS. WzlZzcgm 1-]. Elliot .Eclwm WZSeeger A ORNEY.

Sept. 20, 1955 w. ELLIOT ET AL 2,718,567

PLUGGING SWITCH MECHANISM Filed Jan. 9, 1952 2 Sheets-Sheet 2 l INVENTORS.

BY dwzl l Seeger A ORNEY.

United States Patent PLUGGING SWITCH MECHANISM William H. Elliot, Whitefish Bay, and Edwin Wilbur Seeger, Wauwatosa, Wis., assignors to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application January 9, 1952, Serial No. 265,668 13 Claims. (Cl. 200-87) This invention relates to control devices, and particularly to a switch mechanism for opening a plugging circuit after deceleration of the plugged motor to a predetermined low speed.

An object of the invention is to provide a simple and effective means for preventing the reversal of an electric motor after the plugging thereof to substantially a stopped condition.

Another object of the invention is to provide a plugging switch mechanism wherein the contact actuating component remains in an unactuated position during running operation of the associated motor and is actuated to contact shifting position after plugging of the motor, as distinguished from mechanism wherein the contact actuating component is maintained in actuated position during running operation of the motor and is released to a contact shifting position after motor plugging.

Another object is to provide a plugging switch wherein the motor speed at which the contact portions of the switch are shifted may be easily adjusted to predetermined values without varying the force available for operation of said contact portions.

Another object of the invention is to provide a control device incorporating a substantial number of magnet components which are arranged for minimized effect upon the contact actuating portion of the control device except during movement of certain of said magnet components at a relatively low speed.

A further object is to provide a plugging switch mechanism embodying a statically balanced structure for actuating the contact portions of the mechanism, the contact portions being operable to control plugging circuits for the associated motor during both forward and reverse motor operation.

The mechanism, in general, comprises a first magnet element driven by the motor to be plugged, and a second magnet element magnetically associated with the first magnet element and adapted to actuate an electrical contact means after plugging of the motor and first magnet element to a relatively low speed. The magnet elements are constructed and arranged for minimized magnetic actuation of the second magnet element during operation of the motor and first magnet element at normal running speed, and means are provided to adjust the resistance to movement of the second magnet element for variation of the low motor speeds at which the electrical contact means are operated.

These and other objects and advantages of the invention will be set forth more fully in the following description of the embodiment of the invention illustrated in the accompanying drawings.

In the drawings:

Figure l is a top plan view illustrating the plugging switch mechanism with the cover thereof removed;

Fig. 2 is a longitudinal vertical section taken generally along the shaft portions of the mechanism;

Fig. 3 is a transverse section along line 33 of Fig. 2, with the'stop means being schematically shown to better lit Patented Sept. 20, 1955 ice illustrate the location thereof relative to the weight bars;

Fig. 4 is a schematic illustration showing the elements in a contact shifting position effected by locking of two inner magnets with two outer magnets after deceleration of the inner magnets to a low speed;

Fig. 5 is a view corresponding to Fig. 4 and showing the positions of the elements after the outer magnets have been snapped counterclockwise due to the magnetic attraction of the two succeeding inner magnets; and

Fig. 6 is a wiring diagram of a circuit with which the switch is adapted to be employed.

Referring to the drawings, the plugging switch mechanism is illustrated as mounted in a U-shaped frame or support member 1 having a horizontal base 2 and left and right vertical end walls 3 and 4, respectively. The base portion 2 of the frame has welded thereto a pair of brackets 5 adapted to receive screws for securing an inverted U-shaped cover member 6 in position, and has screwed thereto an upwardly extending frame plate 7 arranged parallel to end walls 3 and 4 and relatively adjacent the latter.

As best shown in Fig. 2, the left frame wall 3 and the vertical frame plate 7 are provided with apertures 8 and 9 adapted, respectively, to receive two axially aligned and independently rotatable shafts 10 and 11. The left shaft 10 is driven by an electric motor, not shown, and the right shaft 11 is connected to operate one of a pair of electrical contact means 12 and 13 after plugging of the motor to cause left shaft 10 to rotate at low speed and consequently effect actuation of the right shaft 11 by means of magnet elements later to be described.

To provide mountings for the magnet elements which operatively relate the shafts, the left shaft 10 is provided with a relatively thick rotor portion 14 and the right shaft 11 is connected at the left end thereof to a magnet cage 15 arranged diametrically outward of the rotor portion 14. The magnet cage 15 comprises a pair of spider or hub members 16 disposed at opposite ends of the rotor 14, and upper and lower magnet mounting bars 17 extending longitudinally of the shafts 10 and 11 and secured by screws to the ends of radially projecting arm portions 18 of the spider members.

The spider members 16 are provided with internal ball bearings 19 and 20 the inner races of which engage, respectively, the main or driving portion of shaft 10 and a short stub portion 21 thereof. In addition, suitable ball bearings 22 and 23 for shafts 10 and 11 are mounted in corresponding bearing housings 24 which are respectively screwed to the left wall 3 of the frame and to the vertical frame plate 7. The bearings 22 and 23, as well as the bearings 19 and 20, permit substantially frictionless rotational movement of the shaft 10 and rotor portion 14, and of the shaft 11 and associated magnet cage 15.

In order to maintain the various components of the mechanism in the desired axial positions, suitable shoulders are provided on the shafts 10 and 11 and suitable spacing washers 25 are provided between the bearings 19 and 22 at the left of the rotor, and between the bearing 23 on frame plate 7 and a washer 26 which is brazed to the right spider 16. The washer 26 facilitates the rigid brazing of the right spider or hub member 16 to a reduced inner end portion of shaft 11 to form a rigid connection therebetween.

The contact means 12 and 13, which are actuated upon movement of the magnet cage 15 and right shaft 11, consist of normally closed sensitive or low pressure switches such as are sold under the trade name of Micro Switch. The switches are bolted to the outer surface of frame plate 7 and are disposed in generally vertical positions on opposite sides of the right end portion of shaft 11. Each of the switches 12 and 13 is constructed with a pivotally mounted vertical arm 27 which is provided at its upper end with a roller 28 for engagement with an upwardly extending actuating crank arm 29. The crank 29 is mounted at the outer or right end of shaft 11 and serves, upon rotation of the magnet cage and shaft 11 in forward or reverse direction, to actuate one of the switches 12 and 13 to open position against the bias of a compression spring (Fig. 1) acting on the arm 27 of the switch.

Referring particularly to Figs. 2 and 3 wherein the magnet portions of the switch operating mechanism are best illustrated, the rotor 14 is provided on its outer surface with four longitudinal grooves which are equally spaced circumferentially of the rotor and in which corresponding base portions of four U-shaped inner magnets 31, 32, 33 and 34 are secured by means of suitable screws. The radially and outwardly extending pole portions of the inner magnets 31 through 34 are arcuate to minimize the air gap between the same and the inwardly extending concave pole portions of corresponding U- shaped outer magnets and 36 which are screwed to the respective mounting bars 17 of magnet cage 15. The inner magnets 31 through 34 are all arranged with their north magnetic poles relatively adjacent one end of the mechanism, whereas the outer magnets 35 and 36 are arranged with their north magnetic poles relatively adjacent the opposite end of the mechanism. Accordingly, a magnetic attraction is present between the outer magnets 35 and 36 and any inner magnets which happen to be adjacent the same. This magnetic attraction is very substantial since the various magnets are formed of a strong permanent magnet alloy such as Alnico.

As will be described more fully hereinafter, the inner magnets 31 through 34 have a relatively small effect upon the outer magnets 35 and 36 when the rotor and inner magnets are in a driven condition resulting from operation of the associated motor at normal speed, but have a strong effect upon the outer magnets after plugging ofthe motor and consequent slowing of the inner magnets to a low speed. When the inner magnets have decelerated to a critical low speed, the outer magnets lock therewith and effect rotation of the magnet cage 15 and shaft 11 to cause crank 29 to operate one or the other of the switches 12 and 13. In order to adjust the critical low speed at which a switch 12 or 13 is operated, means are provided to vary the resistance to movement of the magnet cage. These means preferably comprise an opposed pair of weight bars 37 (Fig. l) which are secured by screws to the ends of relatively short laterally extending arm portions 38 of spider members 16. The weight bars 37 are preferably made of a magnetizable material such as iron in order to increase the magnetic locking force by which the inner magnets effect rotation of the magnet cage 15.

When the weight bars 37 are light, the inertia of the magnet cage is relatively small and the critical speed at which magnetic locking occurs is reached fairly soon after the commencement of plugging action. When additional or heavier weight bars are employed, for example the bars 39 illustrated in Fig. 3, the magnetic locking speed may not be reached until the plugged motor is almost in a stopped condition. By the use of a variety of weights, the mechanism may be adjusted for switch actuation at a large number of motor speeds without variation of the force available for operation of the switches 12 and 13.

In order to achieve static balance in the magnet cage 15 and elements carried thereby, the weight bars 37 are arranged opposite each other, and bars of equal weight are employed to achieve any given switch operating speed. The statically balanced condition eliminates any tendency toward switch operation except under the influence of the inner magnets 31 through 34 as desired.

Referring to Figs. 1 and 3, the angle through which the magnet cage 15, shaft 11, and switch actuating crank 29 may rotate is limited by a pair of rubber stop posts 40 which are mounted on the left frame wall 3 on opposite sides of shaft 10. The stop posts 40 are provided with suitable flange portions 41 disposed, respectively, above weight bars 37 for engagement thereby after forward or reverse rotation of the magnet cage and shaft 11 through a predetermined angle sufiicient to effect actuation of one of the switches 12 or 13.

The operation of the elements thus far described is as follows. Assume that the shaft 10, rotor 14, and inner magnets are being rotated clockwise at the normal running speed of the associated motor, for example at 1750 R. P. M. At this speed the magnet cage 15 and outer magnets remain substantially stationary in the position illustrated in Fig. 3, with any slight magnetic dragging force being counterbalanced by the springs 30 acting on switch arms 27 and thus on the actuating crank 29.

Because of the fact that the inner magnets 31 through 34 are equally spaced circumferentially of the rotor 14, the outer magnets 35 and 36 are sequentially subjected t0 magnetic attractive forces in a clockwise direction, in a counterclockwise direction, and simultaneously and equally in both clockwise and counterclockwise directions. For example, when the inner magnets 31 and 34 are equal distances from outer magnet 35, the resultant magnetic attractive force on magnet 35 will be zero. Upon clockwise rotation of the rotor to place the magnet 34 more closely adjacent the magnet 35, the magnetic force on magnet 35 will be .in a counterclockwise direction. This counterclockwise attractive force will continue until magnet 34 passes beneath magnet 35, at which time the attractive force on magnet 35 will become clockwise and will continue clockwise until the succeeding inner magnet 33 overbalances the attractive force of magnet 34. The described alternating clockwise and counterclockwise forces, as well as the intermittent equal and opposite clockwise and counterclockwise forces, occur rapidly and regularly and result in a minimum of actuation of the relatively high inertia magnet cage 15 during normal running operation of the motor.

Upon plugging of the motor and consequent slowing of the speed of travel of the inner magnets 31 through 34 the alternate clockwise and counterclockwise magnetic attractive forceson the outer magnets 35 and 36 become progressively less frequent until, at a critical speed determined by the various weights 37 and 39, two inner magnets lock with the two outer magnets and effect clockwise rotation of the magnet cage 15. For example, in Fig. the elements are illustrated in the positions assumed after magnetic locking of inner magnets 31 and 33 with the respective outer magnets 35 and 36. When the elements are in these positions, the switch 13 is actuated to its open position and further clockwiserotation of the outer magnets 35 and 36, magnet cage 15, and associated actuating crank 29 is prevented by engagement of the left weight bar with the stop 40 thereabove.

As soon as the magnets 31 and 33 rotate away from the now stopped outer magnets 35 and 36, the next adjacentmagnets 34 and 32 suddenly become operative to attract the outer magnets counterclockwise to the positions illustrated in Fig. 5. The resulting reverse snapping of the magnet cage effects closing of the switch 13 and'momentary opening of the switch 12, with the opening of switch 12' being utilized, if desired, to effect a control function.

The described operation is the same, except for reversal of the operation of the switches 12 and 13, when the motor is running in a reverse direction to effect counterclockwise rotation of the rotor 14.

Referring to Fig. 6, an illustrative control'circuit is schematically shown wherein only one switch 13 is employed to open a plugging circuitand thus prevent motor reversal after plugging of the controlled motor from a high forward speed to a low forward speed. Other control circuits, not shown, may be associated with both of theswitches EIZ-and 13 to effect circuit opening after gnaw? plugging of the motor from either a high forward speed or a high reverse speed.

The illustrated control circuit incorporated a forward contactor 42 and a reverse contactor 43, both of which are only partially shown. The forward contactor 42 is provided with suitable contacts, not shown, adapted when the contactor is energized to effect forward operation of the motor which drives the shaft and rotor 14. The reverse contactor 43, on the other hand, is provided with suitable contacts adapted when the contactor is energized to reverse one winding of the motor and effect rapid plugging thereof to substantially a stopped condition.

In the wiring and operation of the control circuit, the pressing of a normally open start button 44 effects energization of the coil 45 of contactor 42 through a series circuit which includes a normally closed stop button 46 and a pair of suitably energized power lines 47 and 48. Upon energization of coil 45, two sets of normally open contacts 49 and 50 of contactor 42 are shifted to closed position, and a set of normally closed contacts 51 thereof is shifted to open position. As previously indicated, other contacts of contactor 42 also shift to effect forward operation of the motor.

The closing of contacts 49 completes a holding circuit for maintaining energization of coil 45 independently of start button 44, so that coil 45 remains energized after release of the start button 44 and until the stop button 46 is pressed. The closing of contacts 50 completes a circuit from line 47 to line 48 through the coil '52 of a relay 53, causing the relay 53 to shift its normally open contacts 54 and 55 to closed position. When the contacts 54 are closed, an additional path of energizing current for coil 52 is provided in the form of. a circuit comprising the normally closed switch 13 of the described mechanism, a lead 56, contacts 54 and coil 52. The remaining energizing circuit of the control comprises switch 13, contacts 51 of forward contactor 42, contacts 55 of relay 53, and the coil 57 of reverse contactor 43, with the lead 56 being connected at one end to a point between the switch 13 and the contacts 51.

When the forward contactor 42 is energized to effect operation of the motor in a forward direction, the opening of contacts 51 thereof prevents energization of the coil 57 of reverse contactor 43, and consequent plugging of the motor, notwithstanding the closing of contacts 55 of relay 53 as previously described. In order to effect plugging of the motor, the stop button 46 is pressed to deenergize coil 45 of forward contactor 42 and cause contacts 51 thereof to close and complete the energizing circuit for reverse contactor coil 57.

Since contacts 50 of forward contactor 42 are then open, the energization of coil 52 of relay 53 to maintain contacts 55 thereof closed is solely dependent upon the energizing circuit comprising switch 13, lead 56, and contacts 54. As soon as this circuit is opened due to momentary actuation of switch 13 after plugging of the motor to a predetermined low speed as set forth heretofore, the coil 52 of relay 53 becomes de-energized and contacts 55 open to de-energize the reverse contactor 43. Reclosing of switch 13 after the momentary actuation thereof is inoperative to re-energize reverse contactor 43 due to the opening of contacts 54 of relay 53 after de-energization of coil 52 thereof.

Because of the fact that both the forward and reverse contactors are now de-energized, the motor controlled thereby is also de-energized and quickly coasts to a stop without reversing.

The invention provides an extremely simple and effective plugging switch mechanism which may be cheaply manufactured and may be operated with a minimum of upkeep. The use of weights to vary the inertia of the magnet cage permits easy adjustment of the motor speed at which the switch operates without the necessity of uti- 6 lizing relatively complicated apparatus such as governors and the'like.

The term magnetizable, as employed in the appended claims, is intended to apply to an element whether or not the same is already in a permanently magnetized condition.

Various embodiments of the invention may be employed within the scope of the following claims.

We claim:

1. In a plugging switch control device, in combination, a support, a magnetizable member mounted for limited rotary movement on said support, a magnet member mounted for rotation on said support, the mountings for said magnetizable member and said magnet member permitting relative movement of said members to proximate positions for magnetic interaction therebetween, means to impart to said first member a' predetermined resistance to movement sufficiently great to cause the same to remain substantially stationary during relatively rapid movement of said second member therepast and sufficiently small to permit movement of said first member under the influence of said second member during relatively slow movement of said second member, and switch means operatively connected with said first member for actuation thereby to effect a control function upon relatively slow movement of said second member.

2. In a plugging control device, in combination, a frame, a magnetizable member and a magnet member each movably mounted on said frame and disposed for intermittent magnetic interaction with each other, one of said members having an inertial resistance to movement sufficient to cause the same to remain relatively stationary except during movement of the other of said members at a low speed, and electrical contact means operatively connected with said one of said members to effect a circuit controlling function upon movement of said one of said members as the result of the low speed movement of the other of said members.

3. A plugging switch control device comprising, in combination, a support, a first magnetizable member mounted on said support and movable relatively thereto, a shaft adapted to be driven by the motor to be plugged, a second magnetizable member mounted on said shaft and so arranged as to move closely adjacent and past said first magnetizable member upon rotation of said shaft, at least one of said first and second magnetizable members being in a magnetized condition for magnetic interaction therebetween, means including inertia means to impart to said first magnetizable member a predetermined resistance to movement, said predetermined resistance to movement being sufficient to prevent substantial movement of said first magnetizable member by said second magnetizable member except during relatively slow movement of said second magnetizable member effected by rotation of said shaft at a low speed, and electrical switch contact means mounted on said support and operatively connected with said first magnetizable member for actuation thereby only during low speed rotation of said shaft to effect interruption of a plugging operation of said motor.

4. A plugging switch control device comprising, in combination, a support, a first magnet member movably mounted on said support, a second magnet member movably mounted on said support and having at least i one magnetic pole portion so arranged as to be closely adjacent the path of motion of an opposite magnetic pole portion of said first magnet member for magnetic attraction therebetween, said second magnet member .having a resistance to movement sufficiently great to cause the same to remain substantially stationary during relatively rapid movement of said first magnet member past said second magnet member and sufficiently small to permit magnetic locking of said second magnet -member with said first magnet member during relatively slow movement of said first magnet member, and electrical switching means operatively connected with said second magnet member to provide an electrical circuit controlling operation of said switching means as an incident to movement of said second magnet member due to magnetic locking thereof with said first magnet member.

5. A plugging switch control device comprising, in combination, a frame, a first magnet mounted on said frame and adapted to be moved at a speed governed by the speed of the motor to be plugged, a second magnet movably mounted on said frame and having at least one magnetic pole disposed adjacent the path of movement of the opposite magnetic pole of said first magnet for intermittent magnetic attraction between said magnets, means including inertia means to impart a predetermined resistance to movement to said second magnet, said resistance to movement being sufiicient to cause said second magnet to remain substantially stationary except as the result of magnetic locking thereof with said first magnet as an incident to movement of said first magnet at a relatively low speed, and electrical switching means operatively connected with said second magnet for actuation thereby upon movement thereof due to locking with said first magnet for effecting plugging switch control.

6. A plugging switch, comprising a frame, a shaft journalled in said frame and adapted to be rotated by the motor to be plugged, a first magnet member mounted on said shaft for rotational movement therewith, a second magnet member movably mounted on said frame adjacent a portion of the path of movement of said first magnet member, said first and second magnet members being arranged for opposite magnetic poles to move past each other to create intermittent magnetic attraction between said magnet members, means to adjust the inertia of said second magnet member to a predetermined .value effecting maintenance of said second magnet member in a relatively stationary condition during high speed rotation of said shaft but permitting magnetic locking of said second magnet member with said first magnet member upon deceleration of said shaft to a predetermined low speed, and contact means disposed for operation by said second magnet member upon movement thereof due to magnetic locking with said first magnet member.

7. In a plugging switch control device, in combination, a support; a driving magnetizable member movably mounted on said support and adapted to be accelerated and decelerated between a stopped condition and a driven condition; a weighted driven magnetizable member mounted on said support adjacent the path of movement of said driving member for movement thereby between a plurality of controlling positions, at least one of said magnetizable members being in a magnetized condition for magnetic interaction between said members when moved into proximity to each other; and means biasing said driven member to a given controlling position with a force which in combination with the inertia of said driven member is suflicient to prevent movement of said driven member between controlling positions by said driving member during high speed movement of said driving member but permits movement of said driven member to a different controlling position under the influence of said driving member upon deceleration of said driving member to a low speed.

8. A plugging switch, comprising a frame, a rotor journalled in said frame and adapted to be driven by the motor to be plugged, first magnet means mounted on said rotor, a magnet cage substantially freely journalled in said frame and having portions disposed outwardly of said rotor, second magnet means mounted on said magnet cage for magnetic interaction with said first magnet means, weight means to adjust the inertia of said cage to a value effecting maintenance of said cage and second magnet means in a generally stationary condition during normal running operation of said motor but permitting said cage and second magnet means to move with said rotor and first magnet means after plugging of said motor to a predetermined low speed, said second magnet means, magnet cage, and weight means being constructed and arranged to produce a statically balanced condition, and electrical contact means operatively associated with said cage for actuation as the result of movement thereof with said rotor.

9. A control device, comprising a support, a first member movably mounted on said support and adapted to be driven at a relatively high running speed and decelerated therefrom to a stopped condition, a second member movably mounted on said support, first magnet means mounted on said first member, second magnet means mounted on said second member and having spaced magnetic pole portions arranged for movement of the opposite magnetic pole portions of said first magnet means therepast for effecting intermittent magnetic attraction therebetween, at least one of said magnet means including a plurality of independent magnet units spaced from each other to substantially simultaneously attract the other of said magnet means in opposite directions upon movement of said magnet means relative to each other, and control means operatively associated with said second member for actuation upon movement thereof effected by magnetic locking of said first and second magnet means after deceleration of said first member to low speed.

10. A plugging switch, comprising a shaft adapted to be driven by the motor to be plugged, a plurality of permanent magnets mounted circumferentially on said shaft in substantially equally spaced relation and with the opposite poles of each magnet spaced longitudinally of the shaft, a frame, an actuating element movably mounted on said frame, a plurality of permanent magnets mounted on said actuating element and disposed in substantially equally spaced relation adjacent at least some of the permanent magnets on said shaft, the magnetic pole portions of the permanent magnets on said actuating element being spaced longitudinally of said shaft and disposed closely adjacent the paths of movement of the oposite magnetic pole portions of the permanent magnets on said shaft for magnetic attraction therebetween, means to adjust the resistance to movement of said actuating element to a value sutficienly high to prevent substantial actuating element movement during operation of said motor at running speed and sufiiciently low to permit magnetic locking of the permanent magnets on said actuating element with the permanent magnets on said shaft after plugging of said motor and shaft to a predetermined low speed, and a switch mounted for operation by said actuating element upon movement thereof due to slowing of said motor to said predetermined low speed.

11. A plugging switch, comprising a frame, a cylindrical rotor journalled in said frame and adapted to be driven by the motor to be plugged, a substantial number of U-shaped permanent magnets mounted on said rotor and circumferentially thereof, the pole portions of said magnets extending radially outwardly from said rotor and the base portions of said magnets being arranged longitudinally of said rotor and being laterally spaced substantially equal distances from each other, a pair of hub members freely journalled in said frame concentric with said rotor and at opposite ends thereof, a pair of U- shaped permanent magnets mounted on said hub members and diametrically opposite each other, the pole portions of said last named magnets extending inwardly toward said rotor and being arranged closely adjacent the paths of motion of the magnetically opposite pole portions of the magnets on said rotor for magnetic attraction between the magnets on said rotor and the magnets on said hub members, a pair of magnetizable bars mounted on said hub members adjacent the paths of motion of the magnets on said rotor, said bars being arranged diametrically opposite each other and midway between the magnets on said hub members, a switch actuating crank connected to said hub members for actuation thereby upon rotation thereof, a pair of sensitive switches mounted on said frame, said sensitive switches each including arm portions spring biased against opposite sides of said actuating crank, and stop means to limit the rotation of said hub members, said magnets on said hub members, said bars, and said switch actuating arm to a predetermined angle sufiicient to actuate said switches, said switch actuation occurring upon magnetic locking of the magnets on said hub members with the magnets on said rotor after slowing of said rotor to a low speed.

12. A switch comprising, a frame; a rotor rotatably mounted on said frame; at least one permanent magnet mounted on said rotor circumferentially thereof, the pole portions of said magnet extending radially outwardly from said rotor and the base portion of said magnet being arranged longitudinally of said rotor; a hub member movably mounted on said frame concentric with said rotor; at least one permanent magnet mounted on said hub member and having its pole portions extending inwardly toward said rotor and arranged closely adjacent the paths of motion of the magnetically opposite pole portions of the magnet on said rotor for intermittent magnetic attraction between said magnets; a switch actuating crank connected to said hub member for actuation thereby upon movement thereof; electric contact means mounted on said frame and adapted to be actuated by said crank; and means biasing said crank and said hub member to a given position with a force sufficient to maintain said hub member and said crank in substantially said given position during high speed rotation of said rotor but permitting said hub member and said crank to move with said rotor member at a preselected low speed of said rotor to eflect actuation of said electric contact means.

13. A plugging switch, comprising a frame, a magnet cage substantially freely journalled in said frame and having a first magnet member; a rotor journalled in said frame for rotation by the motor to be plugged and including a second magnet member adapted to be rotated past said first magnet member for intermittent magnetic interaction therewith; means biasing said cage to a given position, said biasing means and the inertia of said cage being proportioned to efiect maintenaance of said cage and said second magnet member in a generally stationary condition during normal running operation of said motor and permit said cage and second magnet member to move with said rotor and first magnet member after plugging of said motor to a predetermined low speed; and electrical contact means operatively associated with said cage for actuation as the result of movement thereof with said rotor.

References Cited in the file of this patent UNITED STATES PATENTS 847,597 Onsum et al. Mar. 19, 1907 1,222,720 Bijur Apr. 17, 1917 1,638,718 Vrornan Aug. 9, 1927 1,852,232 Buchhold Apr. 5, 1932 1,960,790 Muftly May 29, 1934 2,141,278 Owens Dec. 27, 1938 2,381,225 Newell Aug. 7, 1945 2,422,973 Martin June 24, 1947 2,432,234 Girard Dec. 9, 1947 2,444,797 Williams July 6, 1948 2,460,015 Jones Jan. 25, 1949 2,596,649 Butler May 13, 1952 FOREIGN PATENTS 584,823 Great Britain Jan. 23, 1947 816,715 Germany Oct. 11, 1951 

